This invention relates to measuring technology and, in particular, to a weight measuring apparatus which is basically employed in processes of automatic dosage measurements and weighing in various branches of industry.
In modern practice development of automatic weighers is directed towards improvement of their design and processing technology, increasing their sensitivity and accuracy of measurements, as well as standardization and normalization of their types and parameters. In this case one of the basic tasks is to provide weight measuring devices featuring a system of remote transmission of data. The main element of such a system is a transducer converting displacements of a weight sensitive element into electrical signals. This system is mounted on some moving part of the weighing scales, e.g. on a beam. It is advisable that these transducers could be installed not only in new weighing equipment but in second hand weighing scales without alterations in their design and unnecessary waste of money.
The main difficulty involved here is the selection of the type of a transducer and matching the working range of the weight scales beam travel and the working range of the moving member of the transducer. This usually requires an intermediate kinematic link and increased accuracy of adjustment, because even insignificant transverse and longitudinal displacements inherent in beam weighing systems result in sharp increase of errors and even jamming.
There are known weighing scales with quadrant or spring balancing. When such weighing scales are used for automatic control they are equipped with potentiometers provided with movable contacts, that is a contact system. Deficiencies of a contact system are commonly known. One of them is insufficient reliability due to existing contact junctions, possible sealing of contacts and their low sensitivity.
There are also known weight measuring apparatuses, wherein the transducer is installed on a metering and balancing mechanism and made as a mag-slip connected to the axis of the pointer of the metering device which moves with respect to the dial face.
Such type of conversion of the pointer angular motion into an electrical signal requires introduction of a receiving selsyn which is to be indentical to the transmitting selsyn and that in general complicates the system of transmission of weighing scales readings.
These weight measuring apparatuses are also deficient in that their resolution is rather low and, consequently, the area of application is limited.
The forementioned weight measuring apparatuses with selsyn remote transmission of readings to obtain a signal to start and stop dosing are provided with contactless discrete pickups interacting with the pointer. For this purpose the pickups are installed on the dial face and the pointer carries plate-screens moving within the pickup response area.
Such positioning of pickups and screens results in considerable additional inertial and frictional load on the pointer axis. There appear slowly damping oscillations of considerable amplitude, which reduce the speed of action of the weight measuring device and distort its readings.
There is also known a weight measuring apparatus comprising an encased weight sensitive element connected to a metallic moving member of a weight sensitive element displacement-to-electric signal transducer, which moves in relation to inductance coils of this transducer secured rigidly on the casing, the winding of the coils is electrically connected to a metering circuit, whereas the winding of the other coil is joined to the power source.
The forementioned apparatus is intended for batch weighing and comprises a weight pan mounted on a leverage which is the weight sensitive element, a metering and balancing device and a weight sensitive element displacement-to-electrical signal inductive transducer.
The forementioned inductive transducer is a differential transformer and the voltage difference in its secondary windings depends on the position of the moving element, that is the position of the armature in relation to the windings, the armature being connected directly to the weight sensitive element.
The drawback of this weight measuring apparatus consists in its insufficient sensitivity and accuracy, which is conditioned by the readings being dependent on uncontrollable mutual motions of the windings of the inductance coils and the armature and caused by the necessity of making the design more complicated and employment of guides.
Besides, in such an apparatus a reverse phenomenon takes place, which consists in the fact that, when the armature moves in the magnetic field of the inductance coils, the resulting electrical interaction brings about a mechanical motive force applied to the armature in the direction opposite to its movement and resisting this movement. Said force is proportional to the current intensity, voltage and distance. That is why its influence has to be reduced at the expense of reducing the output signal, which requires its amplification and employment of complicated secondary equipment, limits the distance between the transducer and the amplifier of the electrical metering circuit and makes the field of application of such weight measuring apparatuses much narrower.